As wireless communications become more digitized, the number of multiple users utilizing the same air space for wireless communications is increasing greatly. In these environments where multiple users are utilizing the same air space, individual communication links are using different frequency channels. For a single user to receive only his communication occurring in one channel, (e.g., the desired channel) the user would ideally employ a perfect filter (i.e., infinite selectivity and zero loss). This perfect filter would allow a user to select only the desired channel, i.e., suppressing all undesired frequency channels, and then perform reception operation, such as amplification, frequency mixing, sampling and digitization. However, in real implementations, filter performance is always a trade-off between loss and selectivity. As a result, filtering and reception are always inter-mixed, to compensate for the signal quality loss associated with filters. The signal is filtered through a low-selectivity, low-loss filter to isolate the desired channel partially, then amplified, then filtered again with medium-selectivity-medium-loss filter to isolate the desired channel further, and amplified again. This process of filtering and amplifying is repeated until appropriate selectivity is achieved, while maintaining minimum levels of loss and noise.
However, reception operations (e.g., amplification, frequency mixing) are also implemented according to their own trade-off: mainly noise versus linearity. When the first filtering step is not selective enough, very large signals located at undesired frequency channels, hereinafter referred to as blockers, may still reach the first amplifying stage and complicate the design and function of this stage. Any blocker reaching the first amplifying stage may be folded into the desired channel by the nonlinearity of the receiver as an inter-modulation product.
FIG. 1 illustrates an exemplary waveform diagram showing a received frequency spectrum at a receiver. Large peaks 104 and 105 represent two potential blockers. Peak 110 contains desired channel 106. Receivers described in the prior art filter out the shorter peaks along with most of blocker 104, by cutting a narrow band (represented by the small bandwidth and High resolution indicators) from the spectrum containing the desired channel 106. A small portion of the blockers remain, however, in the narrow band, and its inter-modulation product 109 is folded onto desired channel 106. When the desired message is decoded, that portion of the message will be degraded or even become non-decodable. For example, in a wireless communication system, such as a cellular telephone network, the communication may be interrupted.
This problem is known as the frequency selective dynamic range problem. Traditionally, improvements are achieved either through higher filter selectivity at constant loss or through better linearity of the reception operation. Therefore, there is a need to improve design trade-offs to reduce the need for highly-selective filters while also reducing the linearity requirements of the reception operations such that a desired channel is digitized not only with minimum inter-modulation, but also with minimum computational overhead. The present invention fulfills this need among others.